CHEMICAL ABUNDANCES OF THE DAMPED Ly α SYSTEMS AT

We present chemical abundance measurements for 19 damped Lyα systems observed with HIRES on the 10m W.M. Keck Telescope. We perform a detailed analysis of every system, deriving ionic column densities for all unblended metal-line transitions. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes which dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary effects often invoked to explain the damped Lyα abundances: (1) nucleosynthetic enrichment from Type II supernovae and (2) an ISM-like dust depletion pattern. Similar to the principal results of Lu et al. (1996), our observations lend support both for dust depletion and Type II SN enrichment. Specifically, the observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest significant dust depletion within the damped Lyα systems. Meanwhile, the relative abundances of Al, Si, and Cr vs. Fe are consistent with both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe and the Mn/Fe measurements from Lu et al. (1996), however, cannot be explained by dust depletion and indicate an underlying Type II SN pattern. Finally, the observed values of [S/Fe] are inconsistent with the combined effects of dust depletion and the nucleosynthetic yields expected for Type II supernovae. This last result emphasizes the need for another physical process to explain the damped Lyα abundance patterns. We also examine the metallicity of the damped Lyα systems both with respect to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and collaborators, i.e., [] = −1.15 ± 0.15 dex. In contrast with other damped Lyα surveys at z > 1.5, we do not formally observe an evolution of metallicity with redshift, although we stress this result is based on the statistics from a small sample of high z damped systems. The damped Lyα systems dominate the neutral gas content of the universe and at high redshift are widely believed to be the progenitors of present-day galaxies (Wolfe et al. 1995). Therefore, one can directly measure the chemical evolution of the early universe by tracing the chemical abundances of the damped systems. Pettini and his collaborators have performed the most extensive surveys on the metallicity of the damped Lyα systems to date (Pettini et al. 1994, Pettini et al. 1997). Working on the premise one can measure accurate column densities of Zn + and …